1. Field of the Invention
The present invention relates to an RF power amplifier useable for an apparatus for transmitting and receiving a radio frequency signal.
2. Description of the Background Art
In general, an RF power amplifier handling radio frequency signals includes, as shown in FIG. 12, an amplification circuit 120 including a plurality of transistors (for example, hetero-junction bipolar transistors) having high radio frequency characteristics, which are connected in parallel, and a bias circuit 110 for supplying a bias current used by the amplification circuit 120. FIG. 12 shows an example of a conventional RF power amplifier having such a configuration (see, for example, Japanese Laid-Open Patent Publications Nos. 2002-9558 and 2003-324325). In the conventional RF power amplifier shown in FIG. 12, a bias current (DC) supply line and a radio frequency signal (RF) input line are connected to a base of each transistor Q.
Conventional RF power amplifiers, represented by the above-described RF power amplifier, have a circuit configuration in which a radio frequency signal and a bias current are supplied to the base of each transistor Q of the amplification circuit 120. With such a circuit configuration, a part of the radio frequency signal leaks from the amplification circuit 120 to the bias circuit 110 via the base connection points (as indicated by the dashed line arrows in FIG. 12). Therefore, when an RF power amplifier having such a configuration is used for an apparatus for transmitting and receiving radio frequency signals, the leak of the radio frequency signals influences the reception of the radio frequency signals by the apparatus as described below.
FIG. 13 is a graph illustrating an exemplary relationship between the transmission frequency/receiving frequency and the output noise in a conventional RF power amplifier (for example, the RF power amplifier shown in FIG. 12). In this specification, the frequency of a radio frequency signal transmitted by the apparatus, i.e., the frequency of a radio frequency signal handled by the amplification circuit 120, will be represented as a “transmission frequency ft”; and the frequency of a radio frequency signal received by the apparatus will be represented as a “receiving frequency fr”. For example, according to the FOMA Standards of NTT DoCoMo, Inc. in Japan, the transmission frequency ft=1950 MHz, and the receiving frequency fr=2140 MHz.
By the leak of a radio frequency signal having the transmission frequency ft from the amplification circuit 120, unnecessary frequency components defined by the receiving frequency fr and the transmission frequency ft of the radio frequency signals are generated in the bias circuit 110. These unnecessary frequency components are a component of a difference frequency fL(=|fr−ft|) between the receiving frequency fr and the transmission frequency ft, and a component of a frequency fH (=2ft−fr) which is lower than the transmission frequency ft by the difference frequency fL. These unnecessary components of the frequencies fL and fH are transferred from the bias circuit 110 to the amplification circuit 120 via a supply path of the bias current (as indicated by the solid line arrow in FIG. 12), and exert a serious influence on the output noise superimposing the band of the receiving frequency fr.